Add default key support to HiveType adapters

hive/lib/src/adapters/ignored_type_adapter.dart

@@ -7,6 +7,11 @@ class IgnoredTypeAdapter<T> implements TypeAdapter<T?> {
   @override
   final int typeId;
 
+  @override
+  dynamic defaultKey(obj) {
+      return null;
+  }
+
   @override
   T? read(BinaryReader reader) => null;
 

hive/lib/src/annotations/hive_field.dart

@@ -22,5 +22,8 @@ class HiveField {
   /// ```
   final dynamic defaultValue;
 
-  const HiveField(this.index, {this.defaultValue});
+  /// Whether use this field as the default key of this type
+  final bool isKey;
+
+  const HiveField(this.index, {this.defaultValue, this.isKey = false});
 }

hive/lib/src/box/box_base_impl.dart

@@ -108,7 +108,7 @@ abstract class BoxBaseImpl<E> implements BoxBase<E> {
 
   @override
   Future<int> add(E value) async {
-    var key = keystore.autoIncrement();
+    var key = _getDefaultKey(value);
     await put(key, value);
     return key;
   }
@@ -117,7 +117,7 @@ abstract class BoxBaseImpl<E> implements BoxBase<E> {
   Future<Iterable<int>> addAll(Iterable<E> values) async {
     var entries = <int, E>{};
     for (var value in values) {
-      entries[keystore.autoIncrement()] = value;
+      entries[_getDefaultKey(value)] = value;
     }
     await putAll(entries);
     return entries.keys;
@@ -183,6 +183,20 @@ abstract class BoxBaseImpl<E> implements BoxBase<E> {
 
     await backend.deleteFromDisk();
   }
+
+  dynamic _getDefaultKey(E value) {
+    var resolved = hive.findAdapterForValue(value);
+    if (resolved == null) {
+      return keystore.autoIncrement();
+    }
+    var adapter = resolved.adapter;
+    var key = adapter.defaultKey(value);
+    if (key == null) {
+      return keystore.autoIncrement();
+    } else {
+      return key;
+    }
+  }
 }
 
 class _NullBoxBase<E> implements BoxBase<E> {

hive/lib/src/registry/type_adapter.dart

@@ -6,6 +6,11 @@ abstract class TypeAdapter<T> {
   /// Called for type registration
   int get typeId;
 
+  // Is called when the value is adding to box without a specific key.
+  dynamic defaultKey(T obj) {
+      return null;
+  }
+
   /// Is called when a value has to be decoded.
   T read(BinaryReader reader);
 

hive_generator/lib/src/builder.dart

@@ -7,8 +7,9 @@ class AdapterField {
   final String name;
   final DartType type;
   final DartObject? defaultValue;
+  final bool isKey;
 
-  AdapterField(this.index, this.name, this.type, this.defaultValue);
+  AdapterField(this.index, this.name, this.type, this.defaultValue, this.isKey);
 }
 
 abstract class Builder {

hive_generator/lib/src/helper.dart

@@ -6,10 +6,11 @@ import 'package:source_gen/source_gen.dart';
 final _hiveFieldChecker = const TypeChecker.fromRuntime(HiveField);
 
 class HiveFieldInfo {
-  HiveFieldInfo(this.index, this.defaultValue);
+  HiveFieldInfo(this.index, this.defaultValue, this.isKey);
 
   final int index;
   final DartObject? defaultValue;
+  final bool isKey;
 }
 
 HiveFieldInfo? getHiveFieldAnn(Element element) {
@@ -19,6 +20,7 @@ HiveFieldInfo? getHiveFieldAnn(Element element) {
   return HiveFieldInfo(
     obj.getField('index')!.toIntValue()!,
     obj.getField('defaultValue'),
+    obj.getField('isKey')!.toBoolValue()!,
   );
 }
 

hive_generator/lib/src/type_adapter_generator.dart

@@ -40,11 +40,17 @@ class TypeAdapterGenerator extends GeneratorForAnnotation<HiveType> {
         ? EnumBuilder(cls, getters)
         : ClassBuilder(cls, getters, setters);
 
+    var defaultKey = getDefaultKey(getters);
+
     return '''
     class $adapterName extends TypeAdapter<${cls.name}> {
       @override
       final int typeId = $typeId;
 
+      @override
+      dynamic defaultKey(${cls.name} obj) =>
+          ${defaultKey};
+
       @override
       ${cls.name} read(BinaryReader reader) {
         ${builder.buildRead()}
@@ -111,6 +117,7 @@ class TypeAdapterGenerator extends GeneratorForAnnotation<HiveType> {
             field.name,
             field.type,
             getterAnn.defaultValue,
+            getterAnn.isKey,
           ));
         }
       }
@@ -126,6 +133,7 @@ class TypeAdapterGenerator extends GeneratorForAnnotation<HiveType> {
             field.name,
             field.type,
             setterAnn.defaultValue,
+            false,
           ));
         }
       }
@@ -151,6 +159,22 @@ class TypeAdapterGenerator extends GeneratorForAnnotation<HiveType> {
     }
   }
 
+  String getDefaultKey(List<AdapterField> fields) {
+    String defaultKey = "null";
+    int keysCount = 0;
+    for (var field in fields) {
+      if (field.isKey) {
+        keysCount++;
+        defaultKey = "obj." + field.name;
+      }
+    }
+    check(
+      keysCount <= 1,
+      'At most one field can have isKey=true',
+    );
+    return defaultKey;
+  }
+
   String getAdapterName(String typeName, ConstantReader annotation) {
     var annAdapterName = annotation.read('adapterName');
     if (annAdapterName.isNull) {